RADIATOR FAN WILL ONLY WORK IF A/C ON

I know the fans work. The car is bubbling over into the overflow and boiling the coolant yet the car temp gauge inside the car says its normal. I have check the fuses and replaced the relay under the hood that is in with the fuses. I have bought an engine coolant temp sensor but idk where it goes. it looks like there are 2 of them on the car at each coolant hose end. I would love for any thoughts on what to do. Im on a budget and want to do this myself if possible. The cooling fan used to turn on when the car was off but i do know they work.